bicycle engine by inducing an appropriately uneven crank angular velocity pattern .... The pedal rates during the submaximal cycling trials at freely chosen pedal rate ..... Broker JP (2003) Cycling power: road and mountain. In Burke. ER ed.
Effect of Chain Wheel Shape on Crank Torque, Freely Chosen Pedal Rate, and Physiological Responses during Submaximal Cycling Ernst Albin Hansen1), Kurt Jensen2), Jostein Hallén1), John Rasmussen3) and Preben K. Pedersen4) 1) 2) 3) 4)
Department of Physical Performance, The Norwegian School of Sport Sciences, Oslo, Norway Team Danmarks Testcenter, University of Southern Denmark, Odense, Denmark Institute of Mechanical Engineering, Aalborg University, Denmark Institute of Sports Science and Clinical Biomechanics, University of Southern Denmark, Odense, Denmark
Abstract The development of noncircular chain wheels for the enhancement of cycling performance has been in progress for a long time and continues apace. In this study we tested whether submaximal cycling using a non-circular (Biopace) versus a circular chain wheel resulted in lower peak crank torque at preset pedal rates as well as resulting in lower pedal rate and metabolic response at freely chosen pedal rate. Ten trained cyclists (meanSD: 273 years of age, 1824 cm tall, 77.57.0 kg of body mass, and peak oxygen uptake of 61.74.4 ml kg1 min1) cycled with a Biopace and a circular chain wheel at 180 W at 65 and 90 rpm for recording of crank torque profiles, and at their freely chosen pedal rate for recording of pedal rate and metabolic response, including oxygen uptake and blood lactate concentration. Crank torque profiles were similar between the two chain wheels during cycling at preset pedal rates. During cycling at the freely chosen pedal rate (being 936 and 934 rpm for the Biopace and circular chain wheel, respectively), blood lactate concentration was significantly different between the two chain wheels, being on average 0.2 mmol l1 lower with the Biopace chain wheel. A musculoskeletal simulation model supported the idea that a contributing factor to the observed difference in blood lactate concentration may be slightly reduced muscle activity around the phase where peak crank torque occurs during cycling with the Biopace chain wheel. In that particular phase of the crank revolution, the observed slightly lower muscle activity may result from larger transfer of energy from the legs to the crank. J Physiol Anthropol 28(6): 261–267, 2009 http://www.jstage.jst.go.jp/browse/jpa2 [DOI: 10.2114/jpa2.28.261] Keywords: biopace, cycling economy, elliptical chain wheel, noncircular chain ring
Introduction More than twenty-five years ago, Okajima (1983) enthusiastically described how his research group at Shimano Corporation aimed to improve the application of the human as bicycle engine by inducing an appropriately uneven crank angular velocity pattern with a non-circular chain wheel. The product of their effort, called the Biopace chain wheel, is best described as a skewed ellipse with nonperpendicular minor and major axes, the latter being close to the crank arms (for a detailed description and depiction, see Cullen et al., 1992, and Hull et al., 1992). Okajima (1983) presented arguments (which will not be repeated here) in favor of the notion that the Biopace chain wheel better optimized the efficiency of the leg muscle work than the circular chain wheel. An increased efficiency would lower the rate of oxygen uptake (VO2) at a given low to moderate speed or power output, which is generally considered advantageous for prolonged cycling (Abbiss and Laursen, 2005; Jeukendrup et al., 2000). However, it has been reported that VO2 measured during submaximal cycling at fixed power output and preset pedal rate was not significantly different between the Biopace and circular chain wheels (Cullen et al., 1992). Thus, the shape of the Biopace chain wheel per se does not appear to improve muscular efficiency. It was also stated by Okajima (1983) that peak joint torques of the legs, as calculated from pedal forces, would be reduced by using a Biopace instead of a traditional circular chain wheel. Unfortunately, data from only one subject were presented. Therefore, the results reported by Okajima (1983) should be taken with some reservation and more comprehensive data on crank torque profile and pedal force are required to make such a conclusion. One study (Neptune and Herzog, 2000) supports the hypothesis of a lower peak crank torque (Tpeak) during cycling with the Biopace versus a circular chain wheel. That study (Neptune and Herzog, 2000) showed lower Tpeak during cycling with an elliptical chain wheel than
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Chain Wheel Shape
with a circular chain wheel. Though not identical to the Biopace, the employed chain wheel in the study by Neptune and Herzog (2000) possessed some of the same characteristics, with its major axis being oriented along the crank arms. The magnitude of Tpeak may be of importance for an individual’s choice of pedal rate during cycling since peak tension development may dominate effort sensation (Mihevic, 1981). A reduction of Tpeak, which occurs, for example, when the crank inertial load is reduced during cycling at a fixed power output (Fregly et al., 1996; Hansen et al., 2002), has therefore been proposed to explain why cyclists choose a slightly lower pedal rate at low compared to high crank inertial load (Hansen et al., 2002). This could potentially reduce the oxygen demand since the freely chosen pedal rate during submaximal cycling is generally higher than the one that results in minimum VO2 (Nielsen et al., 2004). Hence, if Tpeak is indeed reduced with the Biopace chain wheel as implied by Okajima (1983), it is possible that both freely chosen pedal rate and, consequently, internal power (performed on particularly the legs, to move these through each crank revolution) and metabolic response including VO2 (Hansen et al., 2004; Tokui and Hirakoba, 2008), would be lower during cycling with a Biopace chain wheel than with a circular chain wheel. This could reduce the energetic load and perhaps even improve endurance performance during prolonged submaximal cycling. Freely chosen pedal rate has not previously been compared between Biopace and circular chain wheels of the same size. In light of this and the above reasoning it appears therefore that “the Biopace case” may deserve to be reopened. We therefore implemented the present study. The purpose was to test the hypotheses that 1) Tpeak at preset pedal rates of 65 and 90 rpm would be lower when using a non-circular (Biopace) as opposed to a circular chain wheel, and 2) that the freely chosen pedal rate would be lower with the non-circular chain wheel, which secondarily might reduce VO2.
Methods Subjects Ten trained competitive cyclists (273 years, 1824 cm, and 77.57.0 kg) participated in the study, which was approved by the local ethics committee. The subjects signed an informed consent form prior to participation. To minimize the subjects’ focus on their choice of pedal rate, they were not informed of the purpose of the study, but were told that the test session they participated in was merely a pre-test for some additional experiments that they had volunteered for. After the subjects had completed their tests, they were debriefed about the actual purpose as well as the results of the study.
Procedures After a 30-min warm up and a 5-min pause, the subjects performed a 10-min bout at 180 W with either a 52-tooth noncircular Biopace II (CR-BP20, Shimano, Osaka, Japan) or a 52-tooth circular chain wheel, in both cases at a freely chosen
pedal rate. The reason for choosing such a submaximal intensity was that 70% of long (5 h) competitive road races consists of submaximal cycling below approximately 60–70% of peak VO2 (Broker, 2003; Lucia et al., 1999). Thus, despite the submaximal character of this work, it represents a substantial part of the race and is therefore assumed to contribute considerably to the development of fatigue. Then, after a 5-min pause, another 10-min bout with the alternate chain wheel was performed. The order of the chain wheels was counterbalanced to avoid an order effect. During the final 5 min of these 10-min bouts, the means of the following variables were calculated: freely chosen pedal rate, VO2, respiratory exchange ratio, and heart rate. During the final minute, blood lactate concentration ([La]) was measured and the rating of perceived exertion was recorded. After another 5-min pause, the subjects performed two 2-min bouts at 180 W at target pedal rates of 65 and 90 rpm with either the Biopace or the circular chain wheel for determination of crank torque profile characteristics. Immediately after the second 2-min bout, two identical bouts were performed with the alternate chain wheel. The order of the chain wheels was counterbalanced. Finally, in order to obtain a measure of the subjects’ general fitness level they performed after a 5-min pause a single 5-min all-out test at freely chosen pedal rate for determination of the average power output (W5 min), pedal rate, as well as peak values of VO2, respiratory exchange ratio, and heart rate. Peak values of VO2 and respiratory exchange ratio represent the highest means of 30 s sample periods. [La] was measured 1 and 3 min after termination of this all-out test and the highest value was considered to represent peak. This all-out test was performed with a circular chain wheel.
Instrumentation Cycling was performed on an electromagnetic SRM ergometer that was mounted with a Science version of the crank dynamometer (Schoberer Rad Messtechnik, Julich, Germany). The cycle ergometer was adjusted to conform to the measurements of the cyclists’ own racing bicycles and the cyclists used their own cycling shoes and pedals. The SRM ergometer sampled power output and pedal rate at 1 Hz and also allowed measurement of crank torque (sum of the torque applied by left and right leg). For determination of crank torque profile characteristics, crank torque was recorded twice at each 2-min cycling condition, at 500 Hz for 15 s. For each of the two recordings, an average crank torque profile for one crank cycle was calculated by the SRM software. These crank torque profiles were analysed for the three crank torque characteristics of Tpeak, Tmin, as well as crank angle at Tpeak (Hansen et al., 2002). Then, for each subject, cycling condition, and crank torque characteristic, a mean of the two obtained values was calculated, for further analysis. During the submaximal cycling, “gear 9” and the “constant Watt” operating mode of the SRM ergometer were used. When operating in this mode, the ergometer maintains a constant
Hansen, EA et al.
J Physiol Anthropol, 28: 261–267, 2009
power output, regardless of pedal rate. For the 5-min all-out test, the “open end test” operating mode was used with the subjects starting in “gear 9” and being allowed to change gear. In this mode, the power output is changed if either pedal rate is changed or the gear is changed at a constant pedal rate. VO2 and respiratory exchange ratio were determined with an AMIS 2001 metabolic cart (Innovision, Odense, Denmark). Gas analysers were calibrated against certified gases of known concentrations, and ventilation sensors were calibrated with a 3-liter syringe. Heart rate was measured with a Polar S610 heart rate monitor (Electro Oy, Kempele, Finland). [La] was measured in fingertip blood samples using a YSI Model 1500 Sport, with a precision of 2% of reading or 0.1 mmol l1, whichever is larger (YSI, Inc., Yellow Springs, Ohio, USA). Rating of perceived exertion was indicated on Borg’s 6-20 scale (Borg, 1970).
was significantly lower during cycling with the Biopace chain wheel, although the difference in absolute values was small (on average 0.2 mmol l1) (Fig. 1, Table 2). The subjects who had the highest [La] during cycling with the circular chain wheel showed the largest decrease in [La] with the Biopace chain wheel (r0.86, p0.05). The 5-min all-out test confirmed that the subjects were well trained (Peak VO2 60 ml kg1 min1, and W5 min 400 W, Table 3). Power output during the submaximal cycling at freely Table 1 Crank torque profile characteristics obtained during submaximal cycling (180 W) at preset pedal rates with circular and Biopace chain wheel (meanSD) Target pedal rate (rpm)
Pedal rate (rpm)
Statistics Paired t-tests were performed to evaluate differences in measured variables between the Biopace and circular chain wheel at freely chosen pedal rate (StatView 5.0, SAS Institute, Inc., NC, USA). Repeated measures ANOVA with chain wheel and pedal rate as factors, followed by paired t-tests (post hoc), were used to evaluate differences in the measured variables from the part of the study with preset pedal rate. A Pearson product-moment correlation coefficient test was used to analyze the relationship between [La] during cycling with the circular chain wheel and decrease in [La] with the Biopace chain wheel (Excel 2003, Microsoft Corp., WA, USA). The significance level was set at p0.05. Data are presented as meanSD unless otherwise indicated.
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Tpeak (Nm) Tmin (Nm) Crank angle at Tpeak (degrees)
Circular Biopace Circular Biopace Circular Biopace Circular Biopace
65
90
64.61.1 64.71.6 40.43.9 41.14.0 6.01.4 5.71.5 797 808
89.91.5* 88.91.7* 29.02.6* 29.32.0* 4.51.3* 4.70.9* 898* 909*
*Different from 65 rpm (p0.05).
Results The measured Tpeak and Tmin values during cycling at preset pedal rates were significantly lower at 90 rpm than at 65 rpm (p
